It is now well established that dietary PO4 deprivation in both man and animals stimulates the synthesis of the renal calcium-mobilizing hormone, 1,25-(OH)2-vitamin D. However, for the most part, this effect has been studied in young, growing male animals and has not been well described in adult animals of both sexes. The principal objectives of the present studies are to study the time-course of this effect in adult rats of both sexes and to determine whether there is a difference between males and females in their response of 1,25-(OH)2-D synthesis to dietary PO4 deprivation. The results of the present studies are suggestive of an age and/or sex difference in the response of vitamin D metabolism to dietary PO4 deprivation in adult rats. Experiments are currently in progress to document the time-course of the response of vitamin D metabolism to dietary PO4 deprivation in both young and older adult male and female rats. Similar experiments will also be performed in castrated male and female rats and castrated animals given sex steroids.